BORIS Johnson has said Britain should consider military action against the Assad regime in Syria if there is new, verified and incontrovertible evidence that it has used illegal chemical weapons against its own people.

The Foreign Secretary said: “It’s very important to recognise there’s no military solution that we in the West can now impose and we have to be absolutely clear about this, the people listening to us…in eastern Ghouta cannot get the idea that the West is going to intervene to change the odds dramatically in their favour.

"But what we need to ask ourselves as a country, and what we in the West need to ask ourselves, is can we allow the use of chemical weapons, the use of these illegal weapons to go unreprieved, unchecked, unpunished? And I don’t think that we can."

Mr Johnson added: "If there is incontrovertible evidence of the use of chemical weapons, verified by the Office of the Prevention of Chemical Weapons, if we know that it’s happened and we can demonstrate it, and if there is a proposal for action where the UK could be useful, then that we should seriously consider it."

In 2013 David Cameron, the then Prime Minister, lost a vote in the Commonsto instigate possible military action in Syria, joining US-led strikes, by 285 votes to 272.

Asked about whether or not Theresa May supported Mr Johnson’s remarks, her spokesman said the UK Government called on all parties to adhere to ceasefires and cessations of hostilities and to respect humanitarian law that protected civilians.

He explained that UK efforts were focused on de-escalating conflict on the ground and to achieving a political settlement.

Denouncing the use of chemical weapons as abhorrent, the spokesman said: “As the Foreign Secretary said today if there is clear, verified evidence of the use of chemical weapons and a proposal for action where the UK could be useful, then we will look at the range of options.”

Asked if military action was one of those options, he added what was important was where the UK could be useful, stressing how there were no current plans for military action and any such action would need to have “careful consideration and a legal basis”.

In London, Alexander Yakovenko, the Russian ambassador, was called to the Foreign Office to discuss the situation in Syria.

Sir Alan Duncan, the Europe Minister, stressed UK concerns about the crisis in eastern Ghouta and the need for a ceasefire.

A Foreign Office spokesman explained: "Minister Duncan reiterated the UK Government’s deep concern about reports of continued attacks by pro-regime forces in eastern Ghouta, where an estimated 400,000 people have been under siege since 2013, including reports of chemical weapons use.

"Minister Duncan urged Russia to use its influence to ensure the Syrian regime adhered to the ceasefire in order to allow rapid, unimpeded and sustained humanitarian access and non-conditional medical evacuations which are urgently needed across Syria, but particularly eastern Ghouta."

Ahead of the meeting, Mr Yakovenko said: "Russia is firmly committed to UN-led peace process and political settlement. Save Syrians from terrorists and let them decide their future."